Many switches have been developed for measuring the flow of fluid through a pipe. Several such devices rely on the pivotal movement of a rod supported blade which is deflected depending on the amount of fluid flow. For example, in Miller U.S. Pat. No. 3,303,852 issued Feb. 14, 1967, a paddle is displaced due to the flow of liquid, the blade being attached to a rigid rod that pivots in a rubber coupling. The paddle is disposed transversely of the direction of flow so as to move the actuator rod in a counterclockwise direction in response to flow of fluid. The paddle is sufficiently light so that, at higher rates of flow, it deflects into a position out of the flow path to decrease pressure drop through the area of the switch. In Simons, et al. U.S. Pat. No. 4,282,413 issued Aug. 4, 1981, a flow indicator for an automatic sprinkler includes a flexible one-piece plastic vane having a generally circular portion disposed transversely within the pipe. Flow of water through the line causes a magnet at the upper portion of the vane to move into proximity with a sensor to complete an electrical circuit and generate a signal indicating that the water line has been opened. The blade is adapted to move generally parallel to the flow when the line is open.
An additional flow detector employing a pivotal mechanism is shown in Graves U.S. Pat. No. 4,614,122 issued Sept. 30, 1986. This device includes a saddle held in sealing relation to a conduit through an adapter. The adapter includes an aperture that cooperates with an aperture in the conduit through which the actuator of the detector extends. Rod movement dependent upon flow, the rod being spring biased, operates a switch to detect a preset flow condition. See also Miller, et al. U.S. Pat. No. 3,536,873 issued Oct. 27, 1970, which describes a switch which monitors flow of coolant in an engine and provides a warning signal if flow falls below a preset level. This switch employs a diaphragm composed of a material such as an elastomer, and an elongate member pivotally supported, for example, by a washer to provide electrical communication between the elongated member, its electrical contact point and a switch body.
Two other known flow switches include the Q9 and Q10 models sold by the Harwil Corporation of Santa Monica, California. The Q9 includes a reed switch potted in a housing and a bending beam adjacent thereto. The bending beam supports a magnet housing which is caused to bend away from the reed switch housing at the onset of high flow conditions, thereby causing an electrical impulse to indicate the start-up flow condition. The Q10 includes a blade holding a magnet which moves toward a tube-like housing in the flow area, which housing contains a reed switch. The housing also acts as a stop for limiting further blade movement. The Harwil device requires large movement of the magnet to achieve switch closure and would require a different bending blade for each set point.
Flexible blades are shown in Clark, et al. U.S. Pat. No. 4,468,532 issued Aug. 28, 1984. One portion of a conductor is fastened to the housing and another portion is movable when fluid pressure is applied through a passage to force the moveable portion into contact with electrical terminals. This device is designed for switching in a remote, high temperature environment subject to high centrifugal forces which may also be limited by space. Another fluid flow transducer is disclosed in Pisors U.S. Pat. No. 3,629,532 issued Dec. 21, 1971. This device, designed for regulating fuel in feed lines, includes a normally closed resilient moveable contact which is directly impinged by flowing fuel to open an electrical circuit to generate signals which are indicative of the rotational speed of an engine and the frequency and time variation of fuel flow through each fuel line. A center aperture of a contact plate includes the contact surface extending between the center of the aperture and formed integrally in one piece with a contact plate. Electrical current flow is established through movement of the resilient contact surface and is sent to an oscilloscope, a timing light or the like.
Another known flow switch uses a spring loaded piston, rather than a pivoting blade. As the fluid flow rate increases, a piston is pushed back against a spring. A magnet mounted on the piston moves proximately to a reed switch and triggers the switch once the piston travels a sufficient distance.
Each of the foregoing devices suffers from disadvantages which are significant when precise determination of low flow rates is desired. Pivoting systems are particularly troublesome in that they tend to accumulate debris at the pivot area which can cause jamming and prevent proper functioning. Moreover, present designs do not permit the establishment of different flow levels for switch operation. The present invention addresses these disadvantages.